1950 trailer truck airplane - GIF, video, and photos

[Frank S]

Can anyone explain why this idea never, ahem, "took off"?

One of the reasons was the need to transport the empty specialized containers back to point of origin for refilling.
with the advent of cargo planes such as the Military C130 and many others there was no requirement of the specialized containers Palatalized cargo could even be air dropped.
The As64 flying crane could carry a wide variety of suspended cargo. But even those were not as useful as the C47 series Chinooks with these they could either transport cargo inside or up to 47 troops or lift up to 20,000 lbs under belly suspended from 1 or all 3 of the winches
Currently the King of air cargo craft would the Antanov AN 225. But that may be upstaged soon by the stratolaunch maybe not in capacity though.